This invention relates to an active/standby switching system and an active/standby switching method which are capable of preventing occurrence of cell loss at an AAL2 (ATM Adaptation Layer Type 2) layer.
An AAL2 cell assembling/disassembling processor is used In an ATM exchange or a system for carrying out application processing using an ATM cell. The AAL cell assembling/disassembling processor comprises a single processor unit which actually carries out processing. There is no AAL2 cell assembling/disassembling processor having a dual-system structure including two processor units as an active system and a standby system. As the degree of multiplexing is increased and a processing band is broadened, the processing ability of the single processor unit is greatly improved. However, because the AAL2 cell assembling/disassembling processor has such a single-system structure, an influence upon occurrence of system down of the single processor unit becomes more and more serious. In view of the above, there is an urgent demand to achieve an AAL2 cell assembling/disassembling processor having a dual-system structure.
As a first conventional technique similar in technical field to this invention, a “switching device” is disclosed in Japanese Unexamined Patent Publication No. 2002-44090 (JP 2002-44090 A). Th first conventional technique is an invention related to the switching device for switching an active route and a standby route for transmission of an ATM (Asynchronous Transfer Mode) cell between a transmitting unit and a receiving unit. The transmitting unit of the switching device includes ready signal generating means for generating a switching ready signal for requesting switching between the active route and the standby route, packet assembling means which is supplied with upper layer variable-length data to be transmitted and received between the transmitting unit and the receiving unit and converts the upper layer variable-length data into a series of packets to produce a packet stream and which produces a switching position indicating packet in response to the switching preparation signal and inserts the switching position indicating packet into packet streams to be sent through the active and the standby routes at insert positions coincident with each other to produce active and standby packet streams, and a plurality of cell assembling means of a redundant structure supplied with the active and the standby packet streams for Individually multiplexing the active and the standby packet streams to assemble active and standby ATM cells, respectively.
On the other hand, the receiving unit includes a plurality of cell disassembling means of a redundant structure for disassembling the active and the standby ATM cells to reproduce the active and the standby packet streams as reproduced active and reproduced standby packet streams, respectively, and packet disassembling means for detecting as a detected insert position the insert position of the switching position indicating packet in each of the reproduced active and the reproduced standby packet streams and reproducing the upper layer variable-length data from a part of the reproduced active packet stream before the detected insert position and a part of the reproduced standby packet stream after the detected insert position.
The “switching device” disclosed in Japanese Unexamined Patent Publication No. 2002-44090 intends to achieve uninterrupted AAL2 switching but never achieves active/standby switching without cell loss at an AAL2 cell level. The reason is as follows. In an existing active/standby switching method, information necessary to assembling/disassembling of an AAL2 cell (such information will be referred to as “handover information” in the present specification) is omitted upon carrying out active/standby switching so that the cell loss at the AAL2 cell level inevitably occurs after the active/standby switching.
For example, the AAL2 cell has a structure such that user data of a plurality of users are multiplexed in a single cell. As illustrated in FIG. 3, us r data of a single user may extend across two AAL2 cells In the situation where such cross-cell data are present, it is assumed that the handover Information (for example, a sequence number) for confirming the validity of the order of the AAL2 cells is not supplied from a formerly active system to a formerly standby system. In this event, the sequence number of each of the AAL2 cells is unknown so that the order of the AAL2 cells is not guaranteed. This results in a disadvantage that the AAL2 cell can not be assembled.